Recuperative furnaces are well known in the art. They are also known as condensing furnaces. They are distinguished from conventional or non-condensing furnaces by their recovery of a portion of the latent, as well as sensible, heat of the water vapor formed in the combustion process.
Non-condensing furnaces exhaust flue or vent gases at temperatures up to 550.degree. F. As a result, such furnaces produce flue gases in which the water generated by the combustion process remains in the gaseous state; the latent heat of vaporization is not recovered. The efficiency of a furnace which operates in a non-condensing mode generally has a maximum in the range from 75 to 85 percent. On a seasonal basis, this efficiency is reduced because usable heat is also lost up the chimney through a draft hood during the cool-down period at the end of each heating cycle.
Recuperative furnaces, on the other hand, do not employ draft hoods. The flue products are cooled to the dew point while still in the appliance, and some of the latent heat of vaporization is recovered as usable energy. This results in substantially higher efficiencies as less energy is lost out the flue. Vent gas temperatures may be as low as 100.degree. F. and there are few, if any, off cycle losses. Accordingly, depending on the type of condensing or recuperative furnace, efficiencies can be in the low to high 90 percent range.
The condensate produced by a recuperative furnace has a typical pH range of two to six: it is mildly acidic and potentially corrosive. By contrast, normal household waste water tends to be slightly alkaline. Its pH runs on the high side of seven.
The primary difference between the environment in a condensing or recuperative heat exchanger furnace and a conventional furnace is the presence of liquid water. The various gases in the flue gas dissolve in this water to form such acids as carbonic, sulfurous, sulfuric, nitric and nitrous. Dissolved oxygen and carbon monoxide are also present. The acid gases lower the pH of the water and promote acid corrosive attack. The combination of carbon monoxide and carbon dioxide may produce stress corrosion attacking some materials of construction as well. Likewise, sulfurous acid has been shown to promote stress corrosion cracking and intergranular attack of some materials.
In the above-described environment, the use of a plastic, such as polyvinylcloride (PVC), for the flue pipe provides advantages. More specifically, PVC is resistant to the reduced flue gas temperatures which may be in the range from 100.degree. F. to 120.degree. F. Also, PVC is substantially impervious to corrosion from the mildly acidic conditions. Furthermore, PVC is considered easy to work with. Also, PVC is generally a less expensive material than metals that could be used for the flue pipe.
The use of a PVC flue pipe, however, presents a set of particular design parameters or objectives for the connection to a stainless steel exhaust manifold on the recuperative heat exchanger. For example, it is important to maintain internal integrity of the exhaust manifold because holes in it and conventional fasteners such as bolts penetrating through them would be subjected to the mildly acidic environment. Furthermore, it is important that the furnace be as modular as possible so that parts can be disassembled for inspection and/or replacement as necessary. More specifically, as with other parts of the furnace, it is desirable to be able to remove the flue pipe to inspect the interior of the exhaust manifold and take corrective maintenance such as, for example, removing a blockage from the condensate drain and flush system to be described later herein. Also, the junction between the manifold and the PVC flue pipe should provide an airtight seal to prevent the escape of fumes and gases into the furnace room.